Dual magnetic circuit low leakage geophone

ABSTRACT

A geophone for detecting seismic energy wherein a pair of cylindrical magnets are coaxially mounted within a hollow cylindrical, magnetically permeable housing with a first set of like polarity pole faces of the magnets in spaced apart confronting relationship centrally of the housing and the remaining like poles in flux communication with opposite enclosed ends of the housing to form an air gap extending from the confronting poles to the surrounding interior housing wall. A sensing coil of hollow cylindrical shape is mounted for coaxial and resilient reciprocation in the air gap region, and the flux emanating from each of the confronting pole faces is forced by the mutual repulsion between such like poles to assume a radial path effectively traversing the coil, thereby minimizing undesirable flux leakage and consequently increasing the electrical output from the coil.

United States Patent Primary Examiner-Rodney Dv Bennett, Jr. AssistantExaminer-Brian L. Ribando Attarney- Robert Civ Clay ABSTRACT: A geophonefor detecting seismic energy wherein a pair of cylindrical magnets arecoaxially mounted within a hollow cylindrical. magnetically permeablehousing with a first set of like polarity pole faces of the magnets inspaced apart confronting relationship centrally of the housing and theremaining like poles in flux communication with opposite enclosed endsof the housing to form an air gap extending from the confronting polesto the surrounding interior housing wall. A sensing coil of hollowcylindrical shape is mounted for coaxial and resilient reciprocation inthe air gap region. and the flux emanating from each of the confrontingpole faces is forced by the mutual repulsion between such like poles toassume a radial path effectively traversing the coil. thereby minimizingundesirable flux leakage and consequently increasing the electricaloutput from the coil.

PATENIEUauususn 3,593,258

INVENTOR.

CLYDE M. SLAVENS BY flmda ATTORNEY DUAL MAGNETIC CIRCUIT LOW LEAKAGEGEOPHONE The present invention relates to electromagnetic sensingdevices and more particularly to geophones or seismometers employed forgeophysical prospecting.

In this sophisticated and modern method of prospecting, artificiallygenerated seismic energy is introduced into the earth, and the seismicvibrations resulting therefrom are detected by a prearranged group orarray of vibration sensitive instruments. Such instruments, known asgeophones or seismometers, conventionally include a coil suspended forresilient reciprocation within a magnetic field. Relative vibrationbetween the coil and the magnetic force lines in response to seismicvibrations induces a representative voltage in the coil, which voltageis communicated to recording and processing equipment through suitableconnecting cables.

Exemplary instruments employed for this purpose are illustrated in US.Pat. No. 3,4l2,376 and U.S. Pat. No. 3,25 I ,028, the latter of whichhas been assigned to the assignee of the present application. In each ofthese known devices, a cylindrical permanent magnet is disposedcoaxially within a hollow cylindrical housing to establish annular airgaps of magnetic flux in the radial interspaces between these twocomponents. One or more sensing coils are resiliently supported withinthese air gaps for coaxial reciprocation relative to the magnets andhousing. While such instruments have proved satisfactory for theirintended purpose, these and other devices have been known to exhibitslight to moderate flux leakage, i.e., magnetic flux which skirts ratherthan intercepts the sensing coil, thereby reducing the available voltageor electrical signal strength issued by the sensing coil in response toseismic vibrations. Consequently, larger and heavier structures havebeen required in order to boost the signal strength to a level whichwould theoretically be possible with a smaller instrument. In additionto increased structure size and mass, some instruments have beenconstructed with a plurality of air gaps and cooperating sensing coilswhich does provide an increase in the output signal. However, such unitshave the disadvantage of a lower overall efficiency due to theadditional air gap and incur the practical disadvantage of a largernumber of component parts.

While adequate signal strength is obviously a necessary feature of theseinstruments, equally significant consideration must also be given to thesize and mass of each geophone unit due to the peculiar handling,transportation and environmental conditions to which these units aresubjected. For example, the geophone must be of a sufficiently minimumsize in order to be lowered into a small diameter seismic borehole. Atthe same time, the instrument must be rugged in order to resist damageto the sensitive portions of the device during handling and deployment.Suitable shielding of the sensing coil is also an important factor inorder to avoid interference from external or earth originating magneticfields,

Accordingly, one of the primary objects of the present invention is toprovide such an instrument having an increased electrical output orsensitivity without necessitating a corresponding increase in the sizeand mass thereof.

It is the further object of the present invention to provide such ageophone instrument wherein the enhanced signal output is achievedwithout sacrificing instrument features which have become known asnecessary or highly desirable in seismic detectors.

Still another object of the present invention is to provide such ageophone wherein the configuration of the magnetic field structureprovides advantageous shielding of the internal instrument componentsagainst external magnetic fields.

These and other objects and advantages of the invention are achieved bya geophone generally comprising a hollow housing of magneticallypermeable material; a pair of separate magnet members disposed withinthe housing and arranged with a first set or like poles in confrontingrelationship generally centrally of the housing and with the remainingset of like poles contiguous with opposing wall portions of the housing;and a sensing coil supported for resilient reciprocation in an air gapregion of uniform flux concentration surrounding the confronting magnetpoles. By virtue of the confronting relationship between like polarityportions of the magnet members, the combined flux emanating therefrom isforcefully directed in an outwardly extending flux path whichnecessarily intercepts or cuts through the surrounding sensing coil andthereafter exits into the surrounding internal wall portions of thehousing. The air gap region within which the sensing coil is disposed,accordingly has an interior boundary defined by the circumferentialmagnet portions adjacent the confronting poles and an exterior boundaryprovided by the internal wall portions of the housing surrounding theconfronting poles. Due to the configuration of the magnetic circuit thusprovided, little if any magnetic flux is allowed to escape or skirt thewindings of the sensing coil and accordingly maximum utilization of themagnetic energy is achieved for inducing a voltage on the coil inresponse to relative movement thereof. Additionally, in accordance withthe preferred embodiment of the invention, the housing of magneticmaterial wholly surrounds the confronting like poles and sensing coil toachieve highly desirable shielding thereof from external or straymagnetic fields. If external fields are allowed to influence the sensingcoil, it will be apparent that erroneous output signal levels wouldresult.

The invention will be more fully understood and appreciated by referringto the following description of the preferred embodiment thereof, whichdescription is to be taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is an elevation view, partly in section, of the geophoneconstructed in accordance with the present invention; and

FIG. 2 is an elevation view of the geophone as seen from arrows 2-2 ofFIG. 1.

With reference to FIG. I, the invention in its presently preferred formgenerally comprises an enclosed cylindrical housing It] formed of asuitably permeable magnetic material and having enclosed ends I! and 12defining an internal chamber 13', a pair of similar cylindricalpermanent magnets I5 and I6 coaxially disposed in chamber 13 with afirst pair of like polarity poles in spaced apart confrontingrelationship and with the remaining pair of like poles affixed toopposing ends II and 12 of housing I0; and a sensing coil I! having ahollow cylindrical form and being suspended coaxially in the annularregion between magnets 15 and 16 and an internal cylindrical wall 18 ofhousing 10 adjacent to the confronting magnet poles. The cylindricalform of housing 10 adapts the instrument for coaxial disposition orlowering thereof within a small diameter seismic borehole. With thegeophone properly in place at a desired depth below the ground surface,artificially induced or naturally occurring seismic energy is detectedby reason of resulting vibratory movement of coil I7 relative to thefield provided by magnets 15 and I6 and housing I0 which in turnproduces a varying voltage across coil terminals 21 and 22.

As the strength or magnitude of this voltage signal is dependent on theamount of magnetic flux which the coil cuts across during inducedmovement or vibration thereof, the present invention provides in thearrangement of housing 10 and magnets 15 and I6 a magnetic circuit whichmaximizes the number of flux lines intercepted by the sensing coil thusachieving a greater degree of efficiency between the output voltagesignal and the amount of magnetic energy stored by the magnets. Thelatter parameter is a function of the sire and mass of the magneticcomponents employed, and accordingly by virtue of the above notedincreased efficiency, it is possible to produce a more compact andlighter weight instrument while at the same time maintaining thesensitivity thereof. Furthermore, as described herein the configurationof the magnetic circuit is such that a symmetrical flux density gradientis achieved in the air gap region so as to minimize distortion in theoutput signal.

As shown in FIG. 1, by arranging magnets 15 and 16 with a pair of likepole faces 23 and 24 in confronting and proximately spaced apartrelationship, a pair of separate flux paths 26 and 27 are realized.Moreover, in the region of the air gap within which coil 17 is disposed,the flux lines of each of paths 26 and 27 undergo mutual opposition byvirtue of the like polarity confronting poles. Thus, essentially all ofthe flux emanating from pole faces 23 and 24 is forced radiallyoutwardly into cutting relationship with coil 17. Upon outward radialtraverse of the annular air gap, the flux lines communicate withinternal cylindrical wall 18 of housing and are thereupon returned tothe remote like polarity pole faces 3| and 32 via paths 26 and 27.Whereas in the absence of this field shaping feature provided by thearrangement of magnets 15 and 16, the flux lines would emanate andterminate at the axial end portions of pole faces 23 and 24, it will beapparent that the mutual opposition of the like polarity poles forces atleast some of the field lines to emerge from or terminate into theadjacent circumferential edge portions of the magnets. This effect isdiagrammatically illustrated by flux lines 34 in FIG. I.

Also, by virtue of this mutual opposition or repulsion of the fieldlines, a desirable axial symmetry is achieved in the flux densitygradient relative to coil 17. In the absence of such symmetry, there isa tendency of the induced coil voltage to be differentially dependent onthe axial direction of coil movement, thereby resulting in distortion ofthe output signal. it will be appreciated that in the present invention,the flux density gradient is such that sensing coil 17 is subject to thesame proportion of magnetic flux regardless of the direction of axialmovement. Thus, a higher degree of analog fidelity is achieved betweenthe vibration stimulus and the output voltage signal.

In addition to providing means for completing the flux paths 26 and 27,housing l0 advantageously serves to isolate or shield coil l7 fromexternal magnetic fields such as those naturally occurring in the earth.As housing wholly surrounds the sensing portion of the instrument and isofa highly permeable material, a barrier is formed against theseexternal magnetic fields.

For reasons of facilitating construction and assembly of the instrumentand in order to further improve the efficient utilization of magneticflux, housing 10 is formed of a pair of identical cup shaped cylindricalmembers 36 and 37 which are matingly joined at their open ends by anannular pole piece 38. Pole piece 38 is provided with a portionprojecting radially inwardly from the adjacent internal walls of housingmembers 36 and 37 which portion defines internal cylindrical wall 18.Preferably, the axial dimension of wall 18 of pole piece 38 iscoextensive with the corresponding dimension of sensing coil 17 and ofgreater axial dimension than the spacing between pole faces 23 and 24.Thus, pole piece 38 by its inwardly projecting wall I! provides a pathof least resistance to the magnetic field lines from the confrontingpole faces wherein such path is in flux cutting relationship with thecoil. Furthermore, by selecting the relative dimensions of pole piece 38and the spacing between magnet pole faces 23 and 24 such that internalwall is bridges this spacing, the flux lines 34 associated with theouter circumferential edge portions of the magnets are properly directedinto cutting relationship with the sensing coil.

Coil 17 is formed and resiliently suspended within the housing byappropriate means well known to those skilled in the art. The suspensionmeans should be such as to allow adequate axial and resilientreciprocation of coil 17 relative to housing 10, wherein such means arehere diagrammatically illustrated as annular spring elements 41 and 42.These elements may take the form of the annular spring suspensionmembers shown in U.S. Pat. No. 3,251,028 or as shown in U.S. Pat. No.3,4l2,376 for example. The electrical connections to coil 17 shouldlikewise allow the desired resilient reciprocation of the coil and inthis instance coiled conductive leads 43 and 44 are employed betweensensing coil 17 and terminals 2| and 22. Apertures 46 and 47 formed inend housing wall 11 served to insulatingly pass terminals 2i and 22therethrough for connection to an external cable (not shown).

What i claim is:

l. A transducer for converting seismic vibrations into electricalenergy, comprising;

a housing formed of a magnetically permeable material and having asubstantially enclosed internal chamber,

a pair of magnets having a first pair of like poles separately affixedto spaced-apart portions of the interior wall of said housing chamberand a second pair of like poles disposed in proximate and confrontingrelationship and in spaced-apart adjacency with the interior walls ofsaid chamber to define a free space region of magnetic fieldtherebetween, and

coil means resiliently supported in said region in flux cuttingrelationship with the field.

2. The transducer as defined in claim I, wherein said housing is ofcylindrical form having enclosed ends, said magnets are of annular formand the first pair of like polesare individually coaxially affixed tothe interior walls of opposite ends of said housing and the second pairof said like poles are disposed generally centrally of said chamber, andsaid coil means comprising an annular winding coaxially mounted with andbetween said magnets and housing for axial reciprocation relativethereto.

3. The transducer as defined in claim 2, wherein said housing comprisesan annular pole piece disposed circumferentially adjacent the secondpair of like poles and projecting radially inwardly from portions of theinterior housing wall adjacent the axial ends of said chamber to definean annular section of minimum radial separation between said magnets andsaid housing.

4. The transducer as defined in claim 3, wherein said magnets are ofcylindrical form and said housing pole piece is formed with a internalcylindrical wall radially surrounding and axially bridging the secondpair of like poles and said winding being of hollow cylindricalconfiguration and coaxially nested between said pole piece wall and theradially outer walls ofsaid magnets.

S. The transducer as defined in claim 4, wherein said pole piece walland said winding are substantially axially coexten- SIVfi.

6. The transducer as defined in claim 4, wherein said winding has anaxial dimension substantially equal to that of said pole piece wall andspring means mounted between each axial end of said winding and saidhousing to provide resilient axial reciprocation therebetween.

1. A transducer for converting seismic vibrations into electricalenergy, comprising; a housing formed of a magnetically permeablematerial and having a substantially enclosed internal chamber, a pair ofmagnets having a first pair of like poles separately affixed tospaced-apart portions of the interior wall of said housing chamber and asecond pair of like poles disposed in proximate and confrontingrelationship and in spaced-apart adjacency with the interior walls ofsaid chamber to define a free space region of magnetic fieldtherebetween, and coil means resiliently supported in said region influx cutting relationship with the field.
 2. The transducer as definedin claim 1, wherein said housing is of cylindrical form having enclosedends, said magnets are of annular form and the first pair of like polesare individually coaxially affixed to the interior walls of oppositeends of said housing and the second pair of said like poles are disposedgenerally centrally of said chamber, and said coil means comprising anannular winding coaxially mounted with and between said magnets andhousing for axial reciprocation relative thereto.
 3. The transducer asdefined in claim 2, wherein said housing comprises an annular pole piecedisposed circumferentially adjacent the second pair of like poles andprojecting radially inwardly from portions of the interior housing walladjacent the axial ends of said chamber to define an annular section ofminimum radial separation between said magnets and said housing.
 4. Thetransducer as defined in claim 3, wherein said magnets are ofcylindrical form and said housing pole piece is formed with a internalcylindrical wall radially surrounding and axially bridging the secondpair of like poles and said winding being of hollow cylindricalconfiguration and coaxially nested between said pole pieCe wall and theradially outer walls of said magnets.
 5. The transducer as defined inclaim 4, wherein said pole piece wall and said winding are substantiallyaxially coextensive.
 6. The transducer as defined in claim 4, whereinsaid winding has an axial dimension substantially equal to that of saidpole piece wall and spring means mounted between each axial end of saidwinding and said housing to provide resilient axial reciprocationtherebetween.